On 11/04/18 22:07, Ivan Kovachev via dmarc-discuss wrote:
I have three questions for you that I am unsure about and hoping that
someone at Microsoft will be able to help:
First two questions are related to Mimecast acting as inbound security
gateway to O365:
1. When Mimecast acts as inbound gateway solution and it receives an
email, it does DMARC checks and lets the email through to O365
environment. Even if an email passes DMARC checks at Mimecast and the
email is let through, then O365 also seems to also be doing DMARC
checks but both SPF and DKIM fail because of the change that Mimecast
does. As a results DMARC fails. My questions is, what is the best
practice here in this scenario? Is there a way to turn off DMARC
checks at O365? Mimecast suggest that it is whitelisted in O365 but
that means that all the spam will be let through as well.
DMARC checking should only occur at the host referred to be the MX
record as SPF is still relevant for at least some email. I believe
Office 365 has a trusted inbound relays option (i.e. Office 365 trusts
the specified hosts to filter their email) although I can't quickly find it.
Mimecast is apparently unwilling to change their service to stop
damaging incoming messages that don't breach the policies being enforced
(they unconditionally unpack and then repack every message, rather than
only those whose contents they have reason to modify).
2. Would O365 send DMARC reports back to the sender in the above case?
And, if O365 sends DMARC reports back to the sender then emails will
be shown as originating from Mimecast but failing DMARC.
Yes and yes if you've not listed Mimecast as a trusted inbound relay.
(Assuming that the trusted inbound relays setting is not a figment of my
imagination, one would hope that Office 365 would not set feedback in
3. Would O365 do DMARC checks for internal emails ie. O365 tenant
employee to another O365 tenant employee? And would it send DMARC
reports in this case?
Yes and hopefully yes.
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